1. Field of the Invention
The present invention relates to a thermal type expansion valve for a refrigerating circuit of an air-conditioning apparatus, and more particularly, to a thermal type expansion valve that adjusts the flow rate of refrigerant conveyed to an evaporator in accordance with the temperature of the refrigerant discharged from the evaporator toward a compressor.
2. Description of the Related Art
A cross-sectional view of a prior art thermal type expansion valve 4, which is installed in a refrigerating circuit of an automobile air-conditioning apparatus, is illustrated in FIG. 4. The air-conditioning apparatus includes a compressor 1, a condenser 2, a receiver 3, the expansion valve 4, and an evaporator 5.
The compressor 1 is operably connected to an engine by an electromagnetic clutch (not shown) and driven by the rotational force of the engine. The compressor 1 compresses gasified refrigerant sent from the evaporator 5. The condenser 2 then condenses the high temperature, high pressure refrigerant gas through heat exchange with air from outside the automobile and liquefies the refrigerant. The receiver 3 receives and temporarily reserves the liquefied refrigerant cooled by the condenser 2. The receiver 3 incorporates a drier (not shown) to remove moisture and particulate matter from the refrigerant. The expansion valve 4 expands the liquefied refrigerant sent from the receiver 3. This lowers the temperature and pressure of the refrigerant and atomizes the refrigerant. The atomized refrigerant from the expansion valve is vaporized by the evaporator 5 through heat exchange with air sent into a passenger compartment of the automobile.
As shown in FIG. 4, the prior art expansion valve 4 includes a metal main body 6 having a rectangular paralleopiped shape. The main body 6 is provided with a first passage 7, which is connected with the outlet of the condenser 2 via the receiver 3, a second passage 8, which is connected to the inlet of the evaporator 5, and a third passage 9, which connects the outlet of the evaporator 5 with the inlet of the compressor 1. A restricting mechanism 13 is arranged inside the first passage 7.
The restricting mechanism 13 includes an orifice 25, a valve member 27, and a coil spring 29. The orifice 25 is formed in the main body 6 to connect the first passage 7 with the second passage 8. The inlet of the orifice 25 is located in the first passage 7. A valve seat 26 is defined on the area surrounding the inlet. The valve member 27 is urged toward the valve seat 26 by the coil spring 29. The valve member 27 closes the orifice 25 when it abuts against the valve seat 26 and opens the orifice 25 when it is separated from the seat 26.
A controlling mechanism 14 is attached to the upper section of the main body 6. The controlling mechanism 14 is provided with an upper lid 16, a lower lid 18, and a film-like diaphragm 17 made of stainless steel and retained between the upper and lower lids 16, 18. A cylindrical coupling tube 18a is formed integrally with the lower lid 18 and is provided with a threaded outer surface 18b. A pressure chamber 11 is defined in the upper section of the main body 6 and connected with the third passage 9. The pressure chamber 11 opens at the upper surface of the main body 6 and has a threaded inner surface. The controlling mechanism 14 is attached to the upper section of the main body 6 by screwing the coupling tube 18a into the pressure chamber 11. An adhesive agent is applied to the coupled section to prevent the loosening of the coupling tube 18a.
A heat detecting chamber 10, in which saturated gas is sealed, is defined between the upper lid 16 and the diaphragm 17. A pipe 33 is brazed to the upper lid 16 to permit saturated gas to be charged into the detecting chamber 10. After charging the saturated gas, the distal end of the pipe is flattened to seal the detecting chamber 10. The pipe 33 is then permanently closed by soldering or the like to maintain the detecting chamber 10 in a sealed state.
A support hole 32 is formed in the generally center section of the main body 6 and is connected to the third passage 9. An elongated heat detecting rod 21, provided with an integrally formed flange 22 at its upper end, is supported by the inner surface of the coupling tube 18a at the flange 22 and slides in the longitudinal direction. The upper surface of the flange 22 is adhered to the diaphragm 17. The detecting rod 21 extends downward through the third passage 9 from inside the pressure chamber 11. The lower end of the detecting rod 21 is slidably supported by the support hole 32. An elongated actuating rod 24 is supported by the main body 6 and is movable in the longitudinal direction. The upper end of the actuating rod 24 abuts against the lower end surface of the detecting rod 21 and the lower end of the actuating rod 24 abuts against the valve member 27 through the orifice 25.
The liquefied refrigerant drawn into the first passage 7 from the condenser 2 via the receiver 3 expands as it passes through the orifice 25 and is thus converted into a low temperature, low pressure refrigerant mist. The refrigerant mist enters the second passage 8 and is then sent to the evaporator 5 to be gasified by the evaporator 5. The refrigerant gas is then sent to the compressor 1 via the third passage 9 of the expansion valve 4.
The refrigerant gas that passes through the third passage 9 flows into the pressure chamber 11. The temperature of the refrigerant gas in the third passage 9 and the pressure chamber 11 is transmitted to the saturated gas in the detecting chamber 10 through the detecting rod 21, the flange 22, and the diaphragm 17. Therefore, the saturated gas in the detecting chamber 10 expands or contracts in accordance with the temperature of the refrigerant gas passing through the third passage. The pressure inside the detecting chamber 10 is fluctuated by the expansion or contraction of the saturated gas. The diaphragm 17 is displaced upwards or downwards by the pressure fluctuation in the detecting chamber 10. The movement of the diaphragm 17 is transmitted to the valve member 27 through the detecting rod 21 and the actuating rod 24. As a result, the flow rate of the refrigerant mist drawn into the evaporator 5 is adjusted by controlling the area of the orifice 25 opened by the valve member 27 in accordance with the temperature of the refrigerant gas discharged from the evaporator 5 toward the compressor 1.
However, the expansion valve 4 of the prior art having the above-described structure has the following problems:
(1) Since the main body 6 of the expansion valve 4 in the prior art is made of metal, the thermal conductivity of the body 6 is high. Thus, the ambient temperature of the expansion valve 4 is apt to affect the temperature of the refrigerant gas in the pressure chamber 11 through the main body 6. An expansion valve is generally arranged inside an engine compartment, where the heat of the engine and other parts raises the temperature therein. Hence, when heat is transferred between the refrigerant inside the pressure chamber 11 and the saturated gas in the detecting chamber 10, the temperature of the saturated gas in the detecting chamber 10 may become greater than that of the refrigerant passing through the third passage 9. This results in inaccurate control of the area of the orifice 25 opened by the valve member 27 and thus hinders accurate adjustment of the flow rate of the refrigerant drawn into the evaporator 5. In addition, this may cause the valve member 27 to frequently open and close the orifice 25 resulting in frequent temperature fluctuation of the air in the passenger compartment.
(2) The high temperature, high pressure liquefied refrigerant drawn into first passage 7 is converted into low temperature, low pressure atomized refrigerant when it passes through the orifice 25 and flows into the second passage 8. However, the high thermal conductivity of the metal main body 6 permits a relatively high amount of heat transfer between the high temperature liquefied refrigerant in the first passage 7 and the low temperature atomized refrigerant in the second passage 8. This causes a loss of heat energy and degrades the cooling efficiency of the air-conditioning apparatus.
(3) The metal main body 6 is heavy. The expansion valve 4 is generally supported by pipes that are connected to the first, second, and third passages 7, 8, 9. However, the heavy weight of the expansion valve 4 causes vibration to apply a large load on the pipes and raises the possibility of damages inflicted on the pipes. In addition, manufacturing the main body 6 requires metal machining.
If the main body 6 were to be made of a synthetic resin material, the above problems (1) through (3) may be solved. However, since the strength of synthetic resin is low in comparison with metal, the valve seat 26 may be damaged due to repetitive contact with the valve member 27 against the resin seat 26. When the main body 6 is made of metal, the valve seat 26 is generally formed by pressing a steel spherical body, having the same dimension as the valve member 27, against the area surrounding the inlet of the orifice 25. By forming the valve seat 26 in this manner, the space between the valve member 27 and the valve seat 26 is completely sealed when the orifice 25 is closed. However, when the main body 6 is made of a synthetic resin material, the valve seat 26 may not be formed in the above manner. Therefore, a gap may exist between the valve member 27 and the valve seat 26 when the member 27 and seat 26 come into contact with each other, which will hinder complete sealing.
(4) The controlling mechanism 14 is attached to the main body 6 by screwing the threaded outer surface of the coupling tube 18a of the lower lid 18 into the inner surface of the pressure chamber 11. The lower lid 18 may be inexpensively formed through pressing. However, the threaded outer surface 18b of the coupling tube 18a must be machined and thus raises machining costs. Additionally, application of an adhesive agent is required to prevent the threaded outer surface 18b of the coupling tube 18a from becoming loose. Thus, this adds to the steps of the assembly operation.
(5) The lower end of the detecting rod 21 is supported by the support hole 32 formed in the main body 6, and the upper end of the rod 21 is supported by the inner surface of the lower lid's 18 coupling tube 18a, which is screwed to the main body 6. In other words, the lower end of the detecting rod 21 is directly supported by the main body 6 while the upper end section of the rod 21 is indirectly supported by the body 6 through the coupling tube 18a. It is difficult to align the support hole 32, directly formed in the main body 6, with the coupling tube 18a, screwed to the main body 6. This may hinder smooth movement of the detecting rod 21 and may thus interfere with the movement of the valve member 27.
The detecting rod 21 is formed by, for example, machining a cylindrical metal piece. The flange 22 is formed integrally with the upper end of the detecting rod 21. Thus, the outer diameter of the rod 21 differs between sections. This results in a large volume of material being machined for the section having a small diameter and causes a waste of material.
(6) The pipe 33, used to charge saturated gas into the detecting chamber 10 of the controlling mechanism 14, projects upward for a relatively long distance from the upper lid 16. Hence, when installing the expansion valve 4 into the refrigerating circuit, the pipe 33 may be deformed if it is hit against other parts in the engine compartment. This may damage the pipe 33 and cause leakage of saturated gas from the detecting chamber 10. To prevent this, it is necessary to reserve a large space in the engine compartment for the expansion valve 4.
To charge and seal the saturated gas in the detecting chamber 10, the pipe 33 brazed to the upper lid 16 is first washed. Saturated gas is then charged therethrough into the detecting chamber 10. Afterwards, the distal end of the pipe 33 is flattened and soldered. The sealing operation is difficult to automate and thus raises manufacturing costs.